drm/amd/display: fix s2idle entry for DCN3.5+

To be able to get to the lowest power state when suspending systems with
DCN3.5+, we must be in IPS before the display hardware is put into
D3cold. So, to ensure that the system always reaches the lowest power
state while suspending, force systems that support IPS to enter idle
optimizations before entering D3cold.
Reviewed-by: Roman Li <roman.li@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -2904,6 +2904,9 @@ static int dm_suspend(void *handle)
 
 	hpd_rx_irq_work_suspend(dm);
 
+	if (adev->dm.dc->caps.ips_support)
+		dc_allow_idle_optimizations(adev->dm.dc, true);
+
 	dc_set_power_state(dm->dc, DC_ACPI_CM_POWER_STATE_D3);
 	dc_dmub_srv_set_power_state(dm->dc->ctx->dmub_srv, DC_ACPI_CM_POWER_STATE_D3);